Access networks are presently experiencing rapid growth around the world. Both residential and business customers are demanding increasingly higher bandwidths from their Internet service providers who in turn are pressed to implement networks capable of delivering bandwidths in excess of 100 Mb/s per customer. For this application, passive-optical-networks (PON) are particularly well suited as they feature lowest capital-equipment expenditures relative to point-to-point and active optical networks. The books by C. F. Lam, Passive Optical Networks: Principles and Practice, Academic Press, 2007, and by L. G. Lazovsky, N. Cheng, W-T. Shaw, D. Gutierrez, S-W. Wong, Broadband Optical Access Networks, Willey, 2011, and publication by C-H. Lee, W. V. Sorin, and B. Y. Kim, “Fiber to the Home Using a PON Infrastructure”, IEEE J. Lightwave Technol., vol. 24, no. 12, pp. 4568-4583, 2006 give good introduction into this technology.
A passive optical network allows a plurality of users to be connected to a node of a core network (for example, a metropolitan area network). A PON comprises an optical line termination (OLT) located at the central office (CO) and an optical distribution network (ODN) which comprises a plurality of optical links and passive optical components arranged so as to form a point-to-multipoint structure whose root is at the CO of the service provider. At its far end, each optical link may be terminated by a respective optical network unit (ONU). Namely, not all channels have to be terminated for the network to work. The ONUs may be located at user's premises and depending on the location of the optical link end one differentiates Fiber To The Home (FTTH), Fiber To The Building (FTTB), or Fiber To The Curb (FTTC), all commonly referred to as Fiber To The X (FTTX).
In a WDM PON, each ONU communicates with the OLT by using a respective pair of wavelengths: an upstream wavelength for data transmission from the ONU towards the CO and a downstream wavelength for data transmission from CO to the ONU. The wavelengths are generally located on a frequency grid specified by the International Telecommunications Union, in this case Recommendation ITU-T G.694.1. One possible arrangement is that the upstream wavelengths are located in the ITU C band (1531-1570 nm) and the downstream wavelengths are located in the ITU L band (1571-1611 nm). Other possibilities, in which downstream wavelengths are in the E band (1371-1470 nm), for example, are also possible, depending on the specific manufacturer or service provider's use of the installed fiber base. The density of wavelength in the band is specified by the grid frequency separation and the ITU-T G.694.1 recommendation currently specifies 12.5 GHz, 25 GHz, 50 GHz, and 100 GHz grid, with even larger frequency separations, such as 200 GHz are possible and have been used. The grid separation indirectly determines the constraints on the transmitters and receivers used in the OLT/ONU.
The optical components at the ONU end and possibly at the OLT end are fiber-optic transceivers: small packaged optical-electronic modules that are used to simultaneously transmit and receive optical signals at two different wavelengths. Every transceiver comprises optical components detect and generate optical signals, and the electronics to convert these signals to/from digital data stream incoming from network processors. Optical transceivers generally use connectors to mate to optical fibers that provide and take away optical signals. When optical signals coming to the transceiver and being transmitted from the transceiver are encoded on different wavelength, i.e., upstream wavelength different from downstream wavelength, a single fiber and fiber connector is often used. Such transceiver include a duplexer which separates the upstream from the downstream traffic.
In a WDM passive optical network, the ODN typically comprises a “remote node”, a feeder optical fiber connecting the remote node to the OLT and a number of distribution optical fibers connecting the remote node to individual ONUs. The feeder fiber lengths vary depending on the service provider's architecture demands and ranges from 20 km to 80 km. The distribution fibers typically have a length ranging from some tens of meters to several kilometers, depending on the environment (metropolitan or rural) and on the application (FTTX). In the attempt to maintain the access network inexpensive, the remote node is typically kept passive so that no separate power supplies need to be installed to power provided to maintain the network. This means that the remote node contains only passive components.
The function of the remote node is to multiplex and demultiplex WDM signals: The upstream signals from all the OLTs, each with its own frequency are combined at an array-waveguide grating in the remote node into a single fiber where all the signals coexist, but are encoded on amplitude modulated signals at different wavelengths. The typical number of different wavelengths that can be fit into a specific ITU band depends on the frequency separation (grid). Typical numbers M for commercial array waveguide gratings are 16 for 200 GHz, 32 for 100 GHz, 40, and 48 for lower separations. At the other end of the network, the array waveguide grating demultiplexes the WDM signal into M signals at different wavelengths and routes these signals to M transceivers.
Array-waveguide grating (AWG) is ubiquitous in optical networking and is used for filtering, separating, combining, and routing signals of different wavelengths as is well known in the art. Its use and principle of operation is described in publicly available texts, such as, “WDM Technologies: Passive Optical Components” by A. K. Dutta, N. K. Dutta, and M. Fujiwara, published by Academic Press in 2003. It is well known in the art today that AWG temperature variation can be efficiently compensated by using so-called athermal array-waveguide gratings. This technology is described in publicly available texts such as “Recent Progress on Athermal AWG Wavelength Multiplexer” by Shin Kamei published at the Optical Fiber Communications conference in San Diego, Calif. in 2009.
Wavelength division multiplexing in passive optical networks (WDM-PON) is one of the actively investigated as next-generation optical network architecture. WDM-PON provides higher bandwidth per user than any other network architecture and hence potentially offers the lowest cost per unit of bandwidth to the user. However, the key difficulty in such a system has been the cost of the components, particularly arising from the need to transmit light at one wavelength for a specific channel and also receive information at any one of several other wavelengths at the user end in the so-called optical network unit (ONU). WDM optical and optoelectronic components traditionally exhibit high cost, among other issues, due to precise wavelength definitions in such systems. A dramatic cost reduction is achieved by eliminating wavelength-specific transceivers at the ONU in the colorless WDM-PON system, also referred to as a system with wavelength-agnostic transceivers in the ONU.
In a colorless optical network, the wavelengths emitted and received by the transceiver in the ONU are defined in the remote node or the CO rather than in the transceiver at the ONU as is well known in the art—see book by C. F. Lam cited above. One commonly implemented architecture uses a broad-band light source (BLS) to provide spectrally-sliced spontaneous emission to injection-lock to a longitudinal mode of the gain and modulation chip (GMC) as described in the book by L. G. Kazovsky cited above. This solution, however, suffers from high cost of the broadband light source. Further reduction in complexity and cost can be realized by using a self-seeding scheme as described in a publication by E. Wong, K. L. Lee, T. B. Anderson, “Directly Modulated Self-Seeding Reflective Semiconductor Optical Amplifiers as Colorless Transmitters in Wavelength Division Multiplexed Passive Optical Networks”, IEEE J. Lightwave Technol., vol. 25, no. 1, pp. 67-74 (2007).
This self-seeding technique, also referred to as self-tuning technique by some authors, relies on locking the GMC emission to a wavelength specified externally by spectrally slicing the spontaneous emission from the GMC itself, i.e., on injection self-locking. Spectral slicing refers to filtering the GMC spontaneous emission and returning it to the GMC for amplification. If the round-trip loss of the external cavity formed by the GMC, the fiber, and the external mirror is smaller than the unsaturated gain available in the GMC (at given current), lasing threshold will be reached. Above threshold all the additional current fed into the GMC will be converted into light at the emission wavelength, while spontaneous emission at wavelengths other than the selected emission wavelength will be reduced due to gain clamping. Resonators with such long cavities (several kilometers) behave like lasers in that they exhibit clear threshold in the light-current characteristic, but the linewidth and noise of the emitted light are governed by spectral slicing (filtering at the array-waveguide grating) and by thermal nature of the spontaneous emission.
A colorless WDM-PON architecture based on self-seeding is illustrated with the help of FIG. 1 (PRIOR ART). FIG. 1 shows the network components connected into a network 100 comprising a central office 101, feeder fiber 111, remote node 109, distribution fiber 103, and ONU transceiver 104. The central office 101 may comprise, in one location, the components from the remote node 109 and the components from transceiver 104 connected with some small length of optical fiber instead of the distribution fiber 103. In other words, the central office may be a mirror image of the components used on the client side with the difference that the wavelengths emitted from the central office 101 (all the downstream wavelengths ΣλD) are different from the upstream wavelengths ΣλU. The upstream may be located in the C-band, while the downstream may be in L-band.
The remote node 109 comprises an array-waveguide grating 116 with M distribution ports 108 and one common port 112. One of the distribution ports is connected to a distribution fiber 103 for illustration. The common port 112 of the AWG 116 is connected to a 45° Faraday Rotator and a mirror, i.e., a Faraday Rotating Mirror configured to reflect a portion of the spectrally sliced spontaneous emission coming from all the transceivers connected to the distribution ports 108, and transmit a portion to the feeder fiber 111 through a semi-transparent mirror 114. This type of arrangement is published by F. Saliou, P. Chanclou, B. Charbonnier, B. Le Guyader, Q. Deniel, A. Pizzinat, N. Genay(1), Z. Xu, H. Lin titled “Up to 15 km Cavity Self Seeded WDM-PON System with 90 km Maximum Reach and up to 4.9 Gbit/s CPRI Links” in ECOC 2012 Technical Digest, paper We.1.B.6.
It is also possible to use an optical coupler to let a portion of energy passing from the AWG 116 to the trunk fiber and portion to a high-reflectivity Faraday Rotating Mirror in the other branch of the optical coupler. This is not shown in FIG. 1, but is published by Q. Deniel, F. Saliou, P. Chanclou, D. Erasme, titled “Self-Seeded RSOA based WDM-PON Transmission Capacities” in the Technical Digest of the 2013 NFOEC/OSA Conference, paper OW4D.3.
The transceiver connected to the distribution fibers typically comprise a duplexer 110 which separates the upstream and the downstream traffic by directing the downstream optical traffic to the detector and trans-impedance amplifier 124, while the light emitted from the gain and modulation chip 105 passes through a Faraday Rotator 115 before reaching the duplexer 110 and being emitted into the distribution fiber 103. The GMC 105 is a chip with two facets 107 and 125 in which the back facet 107 exhibits high reflectivity coefficient and the front facet 125 a very low reflectivity coefficient. It is also common to curve the GMC waveguide at the point where it reaches the chip edge to reduce the reflectivity of facet 125 even further. In addition, the light passing through the high-reflectivity back-facet reflector 107 is generally coupled to a monitor photodiode and its signal is used to automatically control the output power of the GMC chip. The upstream and downstream digital data transmission are processed in the digital chip 126, in the figure referred to as PHY for physical media adapter. Digital data is encoded into the optical signals by modulating the gain of the GMC 105 using an electrical signal connected to the PHY chip 126. Amplitude modulation means that the symbols are encored into the amplitude of the light being transmitted, such as, low and high power, where the intensity of the output beam is modulated between two values per bit or pulse amplitude modulation (PAM) where more than one power level is used to encode information. Note that duplexer is commonly referred to as a diplexer in the industry. The term diplexer refers to a frequency multiplexer with two frequencies in which two signals of different frequencies travel in the same direction, whereas the term duplexer comes from “full duplex communication” meaning simultaneous bidirectional signal flow over a single path, which is also realized with two different frequencies. Diplexer and duplexer are generally passive reciprocal devices and hence may physically be identical. For the purposes of this application, the term diplexer has the same meaning as the term duplexer.
The principle of light generation involves broadband spontaneous emission generated and intensity modulated by the gain and modulation chip 105 (GMC) which is emitted towards the remote node 109 via the distribution fiber 103. The spontaneous emission is filtered in the AWG 116 and light around a narrow linewidth around one wavelength λU is passed through to the Faraday Rotating Mirror 113/114. The reflector 114 reflects a portion of the incident light back towards the AWG 116 and finally to the GMC 105 via the distribution fiber. The existence of two Faraday Rotators 113 and 115 removes the birefringence in the path of the light between the remote node and the transceiver. Transceiver 104 often is equipped with a heater 120 which is used to increase the temperature of the GMC to broaden and shift the gain spectrum of the GMC.
At present time, there is no commercial deployment of self-seeded WDM-PON as described above. Numerous technical issues have prevented this. The main difficulties can be enumerated as originating form (a) the presence of residual modulation in the seeding light which reduces the link margin, (b) link instability originating from random and time-dependent birefringence of the fibers and components and the high degree of polarization sensitivity of the gain medium, (c) noise inherent in the spectrally sliced light, and (d) large linewidths of the emitted light causing large dispersion penalty.
In addition to the above-mentioned network architecture-related issues, there is a device-related issue that is present today and will be become more so a problem in the future development of WDM-PON systems based on BLS-seeding and self-seeding when channel density increases: It is related to the matching of the longitudinal modes of the GMC 105 to the channel of the AWG 116. Namely, the channel wavelength of an athermal AWG 116 is very temperature stable (˜2 pm/° C.), while the longitudinal modes of semiconductor lasers or amplifiers used in GMC 105 are significantly more temperature sensitive; they move at ˜100 pm/° C. For all ambient temperatures of the remote node 109 and the transceiver 104 (note that they are generally different) one has to have at least one longitudinal mode of the GMC fall within the bandwidth of the AWG 116 channels to achieve high output power necessary for efficient operation of the spectrally-sliced system. In order to achieve stable operation in the external cavity with varying ambient temperature and external cavity loss/dispersion fluctuation, the approach taken in the industry has been to (a) minimize the front facet reflectivity on the GMC 105 and (b) use long gain chips whose mode separation is sufficiently small in wavelength so that at least one GMC mode always appears in the AWG passband regardless of temperature and drive current.
The difficulty with both (a) and (b) is increased cost. Reflectivities lower than 10−4 requires precision coating and angled waveguides, while longer chips consume more of the wafer area. Typical Fabry-Perot lasers are 0.5 mm long, while for channel pitch below 100 GHz, the Fabry-Perot lasers will have to become longer than 2 mm to ensure that more than one mode always appears in the AWG channel passband.
Therefore, an unmet need for a low-cost high-performance WDM-PON solution exists in the industry. This application discloses apparatuses and methods of resolving the device related issue to create a cost-effective WDM-PON solution.